1. Field of the Invention
The present invention relates to a method for producing a toner for developing electrostatic image (hereinafter referred to merely as a toner as the case may be) used to develop an electrostatic latent image in electrophotography, electrostatic recording, electrostatic printing and others, more specifically, a method for producing a toner for developing electrostatic image which is excellent in productivity.
2. Description of the Related Art
In recent years, needs of colorizing images have been increasing for image forming devices using electrophotography, such as a copying machine, a facsimile, and a printer. In color electrophotography, reproducibility of vivid color tones is required, and toner for color which can cope also with printing of highly minute images such as a photograph is required.
Into image forming devices which can cope with color electrophotography are set up toners in three colors of cyan (blue), magenta (red) and yellow (yellow), which are the three primary colors, and a toner in black (black). The four-color toners are overlapped onto each other on a printing paper surface as the need arises to perform color mixing (intermediate colors, or secondary colors), thereby reproducing color tones.
In order to make the resolution of images high, it is suitable to adopt a spherical toner having a small particle diameter. As a method for producing the toner, a polymerization method is suggested.
Examples of the toner producing method based on the polymerization method include suspension polymerization, emulsion aggregation, and dispersion polymerization. For example, in the suspension polymerization, a polymerizable monomer, a colorant and one or more optional different additives are first added so as to prepare a polymerizable monomer composition, and then the polymerizable monomer composition is charged into an aqueous dispersion medium containing a dispersion stabilizer to disperse the composition. Furthermore, a high-speed mixer or the like is used to apply a high shear thereto, thereby forming droplets of the polymerizable monomer composition. Thereafter, the aqueous dispersion medium in which the polymerizable monomer composition, which is in a droplet form, is dispersed undergoes polymerization in the presence of a polymerization initiator. The resultant is then subject to washing treatment (washing, filtration and dehydration) followed by drying so as to yield colored resin particles.
In the case of yielding colored resin particles by a polymerization method, the following great advantages are produced as compared with any conventional pulverization method: at a stage when particles are formed (a droplet-forming stage and a polymerization stage in the polymerization method), spherical colored resin particles having a small particle diameter can be produced and further the particle diameter distribution can be adjusted into a sharper form. In recent years, however, as the level of desires for image printing giving a high resolution and a high image quality has been become higher, attempts for making the particle diameter of toner smaller have been made. About polymerization toner also, a new problem has been pointed out.
As the problem, pointed out is a matter that in a polymerization step in a toner producing method, unnecessary particles having a microparticle diameter are generated as a by-product besides target colored resin particles, whereby bad effects are produced onto the productivity of toner, and printing performances. The micro by-product particles are the so-called submicron-order particles, and are particles containing no colorant (hereinafter referred to as “small diameter microparticles”).
When small diameter microparticles are generated as a by-product in polymerization for producing a toner, a filtering member is choked up with a part of the small diameter microparticles in the washing treatment (washing, filtration and dehydration) of yielded colored resin particles when the particles are separated from the aqueous dispersion medium. For this reason, the efficiency of the washing treatment lowers to result in a drop in the productivity of the toner.
When a toner containing small diameter microparticles is used for printing, the small diameter microparticles adhere easily to members in a developing device since the microparticles have a large adhesive force. The adhering small diameter microparticles accumulate gradually so that filming (adherence) is generated in the members. When filming is generated on a photosensitive member inside the developing device, the surface of the photosensitive member is unsatisfactorily electrified so that a desired electrostatic latent image cannot be formed on the photosensitive member. This causes a fall in printing performance.
A polymerization method makes it possible to produce easily small-particle-diameter colored resin particles having a volume average particle diameter Dv of about 3 to 15 μm. However, as the particle diameter range to be aimed is shifted toward a smaller particle diameter, the range becomes closer to the particle diameters of small diameter microparticles in submicron order as described above. Thus, it becomes difficult to separate the desired colored resin particles from the unnecessary small diameter microparticles. It is therefore desired to develop a toner producing method which is excellent in toner productivity and gives a toner excellent in printing performance by inhibiting small diameter microparticles from being generated as a by-product. In order to respond to the above requirements, various attempts are suggested.
For example, PCT International Publication No. WO2006/013847 discloses a polymerization toner producing method of charging a polymerizable monomer composition containing a polymerizable monomer, a colorant and a charge control agent into an aqueous dispersion medium, stirring the composition, adding thereto t-butylperoxy-2-ethyl hexanoate (trade name: PERBUTYL O, manufactured by NOF Corporation) as a polymerization initiator, making the composition into particles, and adding thereto a hydroquinone compound as a water-soluble polymerization inhibitor (an inhibitor of small diameter microparticle production) before the monomer is polymerized.
Japanese Patent Application Laid-Open No. 2007-52039 discloses a method of producing a polymerization toner made of colored resin particles obtained by polymerizing a polymerizable monomer composition containing a polymerizable monomer and a colorant in an aqueous dispersion medium, using as a polymerization initiator t-butylperoxy-2-ethyl butanoate (trade name: TRIGONOX 27, manufactured by Akzo Nobel N. V.; molecular weight: 188, purity: 98%), which is an organic peroxide having a molecular weight of 205 or less and a purity of 90% or more.
However, the investigation of the inventor has demonstrated that the hydroquinone compound used as a small diameter microparticle inhibitor in PCT International Publication No. WO2006/013847 does not necessarily produce an effect of inhibiting the generation of small diameter microparticles as a by-product. In Japanese Patent Application Laid-Open No. 2007-52039, t-butylperoxy-2-ethyl butanoate, which is used as a polymerization initiator, produces a high effect of decreasing the remaining amount of a decomposition product (volatile organic compound) of the polymerization initiator remaining in the toner, and other products, which cause a bad smell. However, it has been proved that the effect of inhibiting the generation of small diameter microparticles as a by-product is not very high.
Furthermore, when the inventor has investigated a method for inhibiting the generation of small diameter microparticles as a by-product, the inventor has obtained a finding that: in a case where a colorant containing a metal (metal-containing organic pigment) is used to produce a toner, a metal not taken in the pigment species which constitutes the metal-containing organic pigment, which will be referred to as a “free metal” herein after, may become an inducing material which produces small diameter microparticles as a by-product; and further the metal may become a material which blocks the effect of the small diameter microparticles.